Miniaturisation and mass manufacture of portable electronic devices have created a demand for portable power. Sources of power, such as fossil fuels and batteries are respectively increasingly expensive and have limitations in terms of their weight and environmental concerns. Solar cells are another alternative source of portable power although they are commonly fragile, physically large, expensive and require orientation to the sun for maximum efficiency.
Wind energy extraction on a small scale also has limitations. Low power wind generators, under say 3 kW, are commonly of two types:
1. Horizontal axis (propeller) types which are usually mounted rigidly on a pole. Turbines of this type are usually noisy, hazardous to bystanders and take up a large amount of space as the propeller requires orientation into the wind with a vane.
2. Vertical axis wind generators are less common but are once again rigidly mounted, on a tower or bracket and are generally individually fabricated rather than mass-produced. The rotors usually have at least two (usually two or three) identical semi-cylindrical blades or vanes which rotate about a vertical axis. Neighbouring vanes may be connected together or to a central shaft. Alternatively, the inner edges of the vanes may be overlapped to allow the driving fluid to exit a first vane and then pass into a further vane to do further useful work.
This latter type of vertical axis wind turbine is often referred to as a Savonius-type. U.S. Pat. No. 1,697,574A and U.S. Pat. No. 1,766,765A disclose classic Savonious-type rotors. Savonius-type rotors are self-starting however they mostly rely on drag to produce rotation and so are unable to generate blade tip speeds much greater than the driving wind speed. For a given torque output, higher blade tip speeds result in increased output electrical power. They are also relatively inefficient at converting available wind energy into electrical energy. This is because of their reliance on drag wherein while the concave side of a first blade faces the wind creating a driving force, the convex side of another blade is also facing the wind thereby creating a retarding force. Rotation occurs because the convex side of the blade presents less of an obstruction to the wind than the concave side does. Savonius-type rotors are also inherently unbalanced due to the torque produced varying widely with the attack angle of the blade relative to the wind during the rotation cycle. This results in mechanical vibrations in the turbine requiring strengthened support structures, and causes power losses due to parasitic vibrations, increased friction in the bearings, and bearing wear and failure.
An improved Savonius-type rotor is disclosed in U.S. Pat. No. 4,715,776A and U.S. Pat. No. 4,838,757A. This rotor is referred to as a Benesh-type and differs from the Savonius-type in that the blades have an aerofoil (or airfoil) cross-sectional shape. In comparison to the Savonius-type rotor, the Benesh-type rotor is therefore more of a lift-type device and so is theoretically able to rotate at greater tip speeds relative to wind speed. However, the above differences accentuate the unbalanced nature of the Savonius design.
Helical blades are used in some vertical axis wind turbine rotors and can improve the out of balance problem as a part of the helical surface of the blade will always be at an optimal angle of attack to the wind. However, helical blades are more expensive to fabricate than non-helical blades and are not generally capable of cost-efficient moulding techniques such as a single transitional mould release or simple two-part “straight-pull” dies. To minimise eccentric rotation of the shaft due to the aforementioned imbalance, existing vertical axis wind turbines usually have low aspect ratios (rotor height to width) resulting in an undesirable somewhat squat appearance to the turbine. Because of the above limitations, vertical axis wind turbines have not been capable of mass production and existing turbines are not generally portable (that is, easily movable and not requiring permanent mounting) and suitable for low power (say under 3 kW) consumer purposes such as trickle charging batteries.
WO9008881A discloses a vertical axis wind turbine having non-overlapping, non-helical, multi-bladed rotors revolving about (or forming a part of) a stay (a rope or cable supporting the mast) on a yacht. A plurality of separate but coupled rotors is aligned axially along the stay. However, the rotor cross-section profiles disclosed are inefficient with regard to the proportion of available wind energy that they can extract. Improved balance is attempted by increasing the number of blades per rotor which increases the complexity and therefore difficulty of manufacture. Furthermore, because of the lack of blade overlap, the rotors are inefficient and have a stall position that hinders starting in low wind conditions. Also, the wind turbine disclosed requires either replacement of a section of the stay or integration into a stay at the design stage and no retrofitting option that leaves the stay intact is disclosed. The aforementioned disadvantages of non-overlapping blades and the inherent imbalance and low speed capabilities of the Savonius-type rotors also apply to the turbine disclosed in WO9008881A.